Nylon-66 has been widely used as a raw material for chassis-based parts for vehicle engines. In order to manufacture the nylon-66, adipic acid and hexamethylenediamine have been used. For example, adipic acid is conventionally prepared from benzene, which may generate amounts of environmentally harmful byproducts. In the related arts, a method of preparing adipic acid using glucose, which is less harmful to the environment, has been proposed. However, the method may require complicated process such that production costs may increase and may negate economic benefits.